The micro magnetic field sensor is an element widely used in, for example, smart phones, wearable devices and Internet of Things (IOT) devices. The micro magnetic field sensor may also be used in other fields of engineering, science, and industry. For providing a function of magnetic measurement on a modern application, the micro magnetic field sensor has to be highly integrated, have low power consumption and provide correct magnetic force/magnetic field measurement.
In various micro magnetic field sensors, the magnetic field sensor exploiting the Lorentz forces is practical. The reason is that this kind of micro magnetic field sensor can be manufactured in a standard CMOS process. In addition, the resonant magnetic field sensor provides relatively high sensitivity and its outputs may be magnified by an amplifier in response to its quality factor, or Q-factor, therefore provides stronger output signals and higher signal-to-noise ratios. As a result, most new-type micro magnetic field sensor structures exploit the principle of the Lorentz forces and operate under its resonance frequency.
A magnetic field sensor using the Lorentz forces generally comprises a mass block which is suspended in a structure or on a substrate via a spring. When a constant current is applied to the mass block, the current and magnetic forces existing in the earth magnetic field or generated by other magnetic objects generate the Lorentz forces, which move the mass block in a direction perpendicular to the current direction and the magnetic force direction. An electrode for detection forms generally in a comb or finger shape which is staggered with a comb or finger shape formed by an edge of the mass block and maintained at intervals. The space between them is equivalent to a capacitor. The electrode for detection can detect a change in capacitance due to a change in the relative position between the mass block and the electrode for detection caused by the movement of the mass block and generate a detection signal representing the change. The detection signal is converted into a voltage form as an output signal. The generated output signal represents a displacement direction and a displacement amount of the mass block under the influence of the magnetic force, therefore a value of the magnetic force can be calculated on this basis.
The operational principle of the resonant magnetic field sensor is basically the same as that of the magnetic field sensor exploiting the Lorentz forces. In addition, the resonant magnetic field sensor uses a driver circuit to supply a constant current signal to the mass block. The frequency of the current signal is equal to the mechanical resonance frequency of the mass block. The current thus drives the mass block to vibrate at its resonance frequency. When the mass block vibrates at its resonance frequency, the displacement direction and the amount of displacement of the mass block, caused by the Lorentz forces so generated, are detected and are used to calculate the magnetic field applied to the mass block. The intensity of signals generated by a resonant magnetic field sensor is stronger than that by a non-resonant magnetic field sensor.
In the conventional resonant magnetic field sensors, an external oscillator is required to drive the mass block of the micro magnetic field sensor to vibrate at its resonance frequency. In such conventional arts, an external oscillator is used to generate oscillation signals at a constant frequency, so to drive the mass block of the magnetic field sensor to vibrate and to lock the vibration frequency at its resonance frequency. For general introduction and descriptions of the application of such external oscillator and the detection of a magnetic field by having the mass block vibrate at its resonance frequency, the following article may be taken as reference: Dominguez-Nicolas: “Signal Conditioning System With a 4-20 mA Output for a Resonant Magnetic Field Sensor Based on MEMS Technology,” Sensors Journal, IEEE, Vol. 12, No. 5, pp. 935-942, May 2012.
Although known resonant magnetic field sensors may drive the mass block of the magnetic field sensor to vibrate at its resonance frequency, the addition of the external oscillator does not only increase the cost and the volume of the magnetic field sensor but also bring difficulties to the calibration of the resonance structure. One main reason is that the instability of the process in the manufacture of the oscillator would alter the resonance frequency of its resonance structure. As a result, each oscillator provides its particular resonance frequency. Every magnetic field sensor using an additional oscillator needs to be calibrated before putting to use, in order to ensure its mass block may vibrate at its resonance frequency and the vibration is locked to such resonance frequency. In addition, the high Q-factor of micro-electromechanical (MEM) detectors also represents the frequency responsive bandwidth of the detector, used as an oscillator, is quite narrow. For example, if the resonant frequency of an MEM detector is 1 kHz and the Q value is 10,000, then its frequency responsive bandwidth is only 1000/10000=0.1 Hz. This characteristic makes the external oscillator need to perform such a high frequency-stability of hundreds of ppm levels. What's worse, the frequency stability of the driver signals also impacts its amplitude, thereby affecting the resolution of the resulting signals.